vol: Add interfaces for registering RX calls

vol: Add interfaces for registering RX calls

Add VGetVolumeWithCall and VPutVolumeWithCall, to associate RX calls
with volume heavyweight references. Also add the interrupt_rxcall
field to the volume package options structure.

This also adds the VIsGoingOffline function, so a caller can tell when
a volume is going offline.

Reviewed-on: http://gerrit.openafs.org/3215
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 33409b2c849d7fae3b10b3202bb15e338ae5b982)

Change-Id: Ib50a17b3479453bbbaef571711ede7115b17318e
Reviewed-on: http://gerrit.openafs.org/6265
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>